Reactions of Alkenes Flashcards
What is Electrophilic Addition?
A common type of reaction with alkenes in which an electrophole species adds to a π bond.
- The pi bond is broken
- In its place, sigma bonds form to two new atoms or groups of atoms
What is Markovnikov’s Rule?
In the addition of HX, H2O, or ROH to an alkene, hydrogen adds to the carbon of the double bond having the greater number of hydrogens.
Describe a Regioselective Reaction.
An addition or substitution reaction in which one of two or more possible products is formed in preferance to all others that might be formed.
What are the related effects by which alkyl groups stabilize carbocations?
Inductive Effect→ The polarization of the electron density of a covalent bond resulting from the electronegativity of a nearby atom.
Hyperconjugation→ Interaction of electrons in a sigma-bonding orbital with the vacant 2p orbital of an adjacent positively charged carbon.
Describe a Stereoselective Reaction.
A reaction in which one stereoisomer is formed in preference to all others. This reaction may be enantioselective or diasterioselctive, as the case may be.
Enatiomer- Stereoisomers that form Non-superposable mirror images.
Diastereomer- Any stereoisomers that are not eneatiomers
Describe an Anti Stereoselective Reaction.
The addition of atoms or groups of atoms to opposite faces of a carbon-carbon double bond.
What is a Halohydrin?
A compound caontaining a halogen atom and a hydroxyl group on adjacent carbons.
Cl-OH is a Chlorohydrin
Br-OH is a Bromohydrin
Define Syn Stereoselective.
The addition of atoms or groups of atoms to the same face of a carbon-carbon double bond.
Define Anti Stereoselective.
The addition of atoms or groups of atoms to opposite faces of a carbon-carbon double bond.
Descrbe a reaction of the Addition of Hydrogen Halide (HX).
- Convert alkenes to haloalkanes
- Markovnikov Addition
- Carbocaion rearrangements are possible
Mechanism
1-protonation of the alkene π-bond to form a carbocation
2-reaction with the halogen to give the haloalkane.
Ex:
Describe a reaction of the Addition of H2O.
Acid-Catalyzed Hydration
- Used to convert alkenes to alcohols
- Markovnikov Addition
- Carbocation rearrangements are possible
- Only in the presence of an acid catalyst does water add to alkenes resulting in the formation of an alcohol.
Mechanism
1-protonation of alkene π-bond to form a carbocation
2-H2O adds to the substituted carbon
3-take a prton away from the H2O to give the alcohol product
Describe a reaction of the addition of molecular Chlorine (Cl2) or Bromine (Br2).
- Used to convert alkenes to vicinal dihalides
- Addition occurs with anti stereoselectivity; halogen atoms add from the opposite face of the double bond.
- Stereospecific Reaction: Z alkenes give different products then E alkenes.
- Carried out with either the pure reagents or in an inert solvent such as CH2Cl2.
Mechanism
1-attack by the alkene π-bond to give a bridge halogen ion intermediate
2-attacked by the halogen from the opposite side to give the vicinal dihalide
Describe a reaction of the Addition of HOCl or HOBr.
- Used to convert alkenes to halohydrins
- Markovnikov Addition (OH adds to the substituted Carbon)
- Anti Addition stereoselective
- Stereospecific: Z alkenes give different products the E alkenes
Mechanism
1-attack by the alkene π-bond to give a bridged halogen ion intermediate (rearrangments do not occur)
2-attacked by H2O from the opposite side to give a halide ion intermediate
3-loss of proton from H2O to give the halohydrin
Describe an Oxymercuration/Reduction Reaction.
- Oxymercuration followed by reduction results in hydration of the C-C double bond
- Markovbnikov Addition (OH bonds to the more subsituted carbon)
- Anti Addition Stereoselectivity
- Occurs without rearrangment, unlike acid-catalyzed additions of water to alkenes
Mechanism
1-attack by the alkene π-bond to give a bridged mercurinium ion intermediate.
2-attack by H2O to the more substituted carbon from the opposite side to open the three membered ring
3-loss of proton by H2O to give the alcohol
4-Reduction of the C—AgOac bond by NaBH4 to give a C—H bond
Describe a Hydroboration/Oxidation reaction.
- Used to convert alkenes into alcohols
- Non-Markovnikov Addition
- Syn Addition Sterioselective
- Stereospecific (cis alkenes give different products than trans alkenes)
- Rearrangments do Not occur
Describe an Oxidation with OsO4 reaction.
- OsO4 oxidizes an alkene to a Glycol, a compound with -OH gorups on adjeacent carbons.
- Syn Addition without rearrangment
- Stereospecific (cis alkenes give different products then trans alkenes)
- Reagents: OsO4 in NaHSO3/H2O
Describe an Ozonolysis reaction.
Oxidation with O3
- Treatment of an alkene with ozone followed by a weak reducing agent cleaves the C=C and forms two carbonyl groups in its place.
- Uses O3 and Dimethyle sulfide [(CH3)2S Reducing agent]
Mechanism
1-formation of a ozonide intermediate
2-reduction using (CH3)2S to give carbonyl products
Describe the addition of H2
Catalytic Reduction
- Used to reduce alkenes to alkanes
- Syn Addition without rearrangment
- If one side of the C-C double bond is more hindered, then the reaction is stereoselective for the less hindered side
- Uses H2 and a metal catalyst (Pt,Pd, or Ni)
Mechanism
1-H and alkene are absorbed on the metals surface
2-one H is transfered to the alkene forming a new C-H bond
3-a 2nd C-H bond is formed and the alkane is desorbed
